A source follower is often used as a voltage buffer to drive subsequent circuits with a relatively low source impedance, especially in high-frequency applications. Voltage buffer circuits have a high input impedance and a low output impedance so that they can be utilized to drive subsequent circuits which have lower impedances than that which an input circuit without a buffer can drive. As such, the buffer can be put in-between the load circuits and input circuits so that the buffer feeds required current to the load circuits. However, complimentary metal oxide semiconductor (CMOS) source followers, such as NMOS and/or PMOS source followers, unlike bipolar emitter followers, have significant attenuation owing to body effect and output impedance of the current source, among other things.
Body effect relates to the effect that a substrate and variations therein have on a threshold voltage, among other things, of a CMOS device (e.g., a transistor). Body effect gives rise to attenuation, and for a source follower circuit with an open load, this can be expressed as gm over gm+gmb, where gm is the transconductance of a circuit. Transconductance corresponds to the ratio between an output current and an input voltage. If an input voltage alternates by one milli-volt, for example, then an output voltage is given by gm over gm+gmb times a milli-volt, due to which the output voltage will be less than a milli-volt.
Additionally, in short-channel CMOS followers, the impedance of the current sources can be relatively small, and the gain of the differential follower is thereby further degraded. If used as a driver to a matched load, for example, there can be an inherent voltage attenuation of 6 dB or more. Similarly, if a CMOS source follower is used to drive an external load (e.g., a 50 ohm load), then considerable attenuation may be experienced in the signal. The above impairments related to CMOS buffer implementations further increase the attenuation. Despite these problems, source followers are widely used as buffers, especially were OP-AMP or feedback amplifier based techniques cannot be employed due to their inadequate frequency response performance.